Slide core coil for a tuned circuit having a constant selectivity



April 3, 1951 T. A. SPOOR SLIDE CORE COIL FOR A TUNED CIRCUIT HAVING A CONSTANT SELECTIVITY Flled May 2, 1946 INVENTOR.

THEODORUS ANTONIUS SPOOR ATTORNEY Patented Apr. 3, 1951 SLIDE CORE COILFOR A TUNED CIRCUIT HAVING A CONSTANT SELECTIVITY Theodor-us Antonius Spoor, Eindhoven, Netherlands, as'signor, by 'mesne assignments, to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application May 2, 1946, Serial No. 666,562 In the Netherlands January 5, 1942 Section 1, Public Law 690, August s, 1946 Patent expires January 5, 1962 3 Claims. (Cl. 171-119) The invention relates to slide-core coils with which the coupling between the coil and a conducting surface arranged in the neighbourhood thereof varies due to the displacement of the core.

As is well-known, slide-core coils are utilized in radio-receiving apparatus and more particularly for tuning the oscillatory circuits to the desired wavelength. They generally consist of a winding provided on a hollow cylinder and of a core pressed from high-frequency iron free of losses, which core is slidable in this cylinder. By causing the core to slide into the cylinder to a higher or less high extent it is possible to adjust the selfinduction of the coil to the desired value. If a conducting metal surface is present in the neighbourhood of such a slide-core coil, the displacement of the core entails a change in the coupling between the coil and the surface. When the tightness of the coupling increases the intensity of the eddy currents induced in the surface also increases, which causes an increasing extra damping of the coil whilst moreover the selfinduction of the coil varies. This variation is the greater the more the coupling between the coil and the conducting surface increases due to the displacement of the core. It has been found experimentally that the ratio between the selfmduction and the loss resistance of the coil, which ratio is decisive for the selectivity if the coil is combined with a condenser to form an oscillatory circuit, becomes less satisfactory when the core approaches the conducting surface.

According to the invention, the said ratio is improved by providing between the coil and the conducting surface a short-circuited winding made of a material of higher conductivity than the material of the conducting surface.

In the construction of slide-core coils for radio-receiving apparatus attempts are generally made, in connection with a linear scale, to obtain a square relation between the selfinduction and the adjustment of the slidable core so that the selfinduction increases the more the greater is the extent to which the slidable core is caused to slide into the coil. It will be clear, however, that this square relation is disturbed the more the greater is the length by which the core penetrates into the coil and the tighter is the coupling with a conducting surface present in the neighbourhood, for example the chassis, for the intensity of the eddy currents induced in the conducting surface steadily increases in this case so that the magnetic field of the coil is weakened which implies a steadily increasing 2 decrease of the selfinduction which neutralizes, or at least partly neutralizes, the increase of the selfinduction due to the further penetration of the core into the coil.

According to the invention, this drawback may also be obviated by surrounding the coil by a screening cylinder which extends to the conducting surface and which acts at the same time as a short-circuited winding for the reduction of the extra-damping of the coil.

The invention will be explained more fully with reference to the accompanying drawing, in which Fig. 1 illustrates the invention employing a short circuited winding; and

Fig. 2 illustrates a modificationemploying a screening cylinder. I

In Fig. 1 of this drawing I. denotes a hollow cylinder of insulating material having provided on it a coil winding 2. The coil cylinder I is secured by means of supports 3 to an iron chassis plate 4 of a radio-receiving apparatus, a core 5 being arranged so as to be slidable in the coil cylinder. The greater the length by which the core 5 is caused to slide into the coil the tighter is the coupling between the winding 2 and the chassis plate 4 with the result that the intensity of the eddy currents induced in the plate increases.

According to the invention, there is arranged between the chassis plate and the coil winding 2 a short-circuited winding 6 made of a material, for example copper, which has a higher conductivity than the iron chassis plate. The short-circuited winding has a screening efiect so that at the place of the chassis the strength of the alternating field and therefore also the intensity of the eddy currents induced therein decrease. It is true that now eddy current losses are also liable to occur in the short-circuited winding but it has been found that if this winding is made of a material of higher conductivity than the iron chassis plate, the ratio between selfinduction and loss resistance is more favorable than in the absence of the short-circuiting winding.

In the above it was assumed that the, iron core is caused to slide into the coil in the direction of the arrow. It is, of course, also possible to cause the core to move in the opposite direction through a hole in the chassis.

It is advisable to arrange the short-circuited winding as close to the chassis as possible and to assure that the coupling between the core and the short-circuited winding is great with respect to the coupling between the core and the chassis.

Fig. 2 represents a modified embodiment of the invention wherein the short-circuited winding is constituted by ajscreening cylinder I'which surrounds the coil and which extends to the chassis plate 4. Such a screening cylinder has the advantage that the course of the selfinduc tion is less influenced than witha single short; 1f circuited winding since, independentlyoff the. adjustment of the core 5, the decrease of the selfinduction is always the same.

I claim: T

1. A variable high-frequency inductance tuning system comprising an electrically conducting base member, an electrically insulating hollow coil supporting member secured tosaid base member, a coil winding arranged, on said coil supporting member and spaced fromgosaid base member, a core member of magnetic material adapted to enter one end of said coil overa wide range, thereby permitting tuning of the system on divergent wave-lengths, and a portion of a short circuited' electrical conductor of higher electrical conductivity than the conductivity of said base member positioned in the space between the coil' winding and said base member remote from the end of the coil into which the core enters for reducing'eddy-current losses in said base member and to maintain constant damping of said coil irrespective of the position of said core with respect to said coil winding and said base member.

2. A'va'riable'high-frequency inductance tuning system comprising an electrically conducting base member, an electrically insulating hollow winding and said base member remote from the end of the coil into which the core enters for reducing .eddy current losses in said base member and to maintain constant damping of said coil irrespective of the position of said core with respect to said coil winding and said base mem- '.Z1:variable high-frequency inductance tunin system comprising an electrically conducting base member, an electrically insulating hollow'coil supporting member secured to said base member," afcoil winding arranged on said coil supporting member and spaced from said base member,fa core. member of magnetic material adapted to enter one end of said coil member over a wide. range, thereby permitting tuning oi.

the" system on divergent wave-lengths, and a shield of material having a higher electrical conductivity than the conductivity of said base member surrounding said coil winding and extending to said base member for reducing eddycurrent losses in said base member and for maintaining constant damping of said coil irrespective of the position of said core with respect to said coil winding and said base member.

THEODORUS ANTONIUS SPOOR.

EFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES, PATENTS Number Name Date 2,038,294 Johnson Apr. 21,1936 2,298,275 B'ohren- Oct. 13, 1942 2,317,724 Bergtold Apr. 27, 1943 2,357,442 I Dodington Sept. 5, 1944 2,361,187 Foster Oct. 24, 1944 2,388,295 Shea Nov. 6, 1945 2,417,188 Clark Mar. 11, 1947 FOREIGN PATENTS Number a Country Date 330,914 Great Britain June 12, 1930 

